An electric power steering apparatus that energizes a steering apparatus of a vehicle by using a rotational torque of a motor as an assist torque, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus (EPS) performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of duty command values of a pulse width modulation (PWM) control.
A general configuration of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft) 2 connected to a handle (steering wheel) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 are provided with a torque sensor 10 for detecting a steering torque Th of the steering wheel (handle) 1 and a steering angle sensor 14 for detecting a steering angle θ, and a motor 20 for assisting the steering force of the handle 1 is connected to the column shaft 2 through the reduction gears (gear ratio “n”) 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command based on a steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 based on a voltage control command value Vref obtained by performing compensation and so on with respect to the current command value in a current control section. Furthermore, the steering angle sensor 14 is not always necessary and it is possible to remove.
A controller area network (CAN) 50 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN. Further, a Non-CAN 51 is also possible to connect to the control unit 30, and the Non-CAN 51 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 50.
The control unit 30 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 that is the target value of the current supplied to the motor 20 based on the steering torque Th and the vehicle speed Vel and by means of an assist map and so on. The current command value Iref1 is added in an addition section 32A and then the added value is inputted into a current limiting section 33 as a current command value Iref2. A current command value Irefm that is limited the maximum current, is inputted into a subtraction section 32B, and a deviation I (=Irefm−Im) between the current command value Irefm and a motor current value Im that is fed back, is calculated. The deviation I is inputted into a PI control section 35 serving as the current control section. The voltage control command value Vref that characteristic improvement is performed in the PI control section 35, is inputted into a PWM control section 36. Furthermore, the motor 20 is PWM-driven through an inverter 37 serving as a drive section. The current value Im of the motor 20 is detected by a motor current detector 38 and is fed back to the subtraction section 32B. In general, the inverter 37 uses EFTs as switching elements and is comprised of a bridge circuit of FETs.
Further, a compensation signal CM from a compensation signal generating section 34 is added in the addition section 32A, and the compensation of the system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation signal generating section 34 adds a self-aligning torque (SAT) 343 and an inertia 342 in an addition section 344, further adds the result of addition performed in the addition section 344 and a convergence 341 in an addition section 345, and then outputs the result of addition performed in the addition section 345 as the compensation signal CM.
The CPU (a micro-computer or the like) of such the electric power steering apparatus generates the voltage control command value for controlling the motor by means of the PI control as described above. The gain of the PI control is adjusted to an appropriate value for each vehicle type.
If the gain of the PI control becomes high, the noisy sound and the vibration caused due to a noise are generated. Accordingly, it is necessary to restrict the gain of the PI control no to generate the noisy sound and the vibration. However, if the gain of the PI control is restricted, the frequency characteristic for the current control falls and it is impossible to enhance a responsibility of the steering assist. Further, even if the gain of the PI control is sufficiently fallen, it is also impossible to fully avoid the vibration in the vicinity of a resonance frequency of the steering system and to necessarily get a good steering feeling.
As an apparatus for resolving the above problems, for example, Japanese Published Unexamined Patent Application No. 2006-188183 A (Patent Document 1) is proposed. That is, the electric power steering apparatus disclosed in Patent Document 1 comprises a vibration detecting means to detect a vibration of an operating member and a gain changing means to make at least any one of a proportional gain and an integral gain of a PI control to fall when the vibration is detected by the vibration detecting means.